Directed by Olivier Assayas (“Clouds of Sils Maria”), “Personal Shopper” is the story of Maureen (Kristen Stewart, “Certain Women”), an American expat living in Paris and working for a wealthy model as a personal shopper and assistant. Maureen is grappling with the recent death of her twin brother, Lewis, of a genetic heart condition. The siblings believed that they both had connections to the spirit world, so after his death, Maureen believes Lewis is attempting to contact her from beyond the grave. She also begins receiving messages from an unknown number, someone who knows intimate details of her life, and is strangely transfixed by them.

The film is difficult to categorize. It’s at once a tense psychological thriller, an understated exploration of grief and loneliness and a horror movie. Really, the film is a study in tonal shifts, some of which are more effective than others. Assayas attempts to balance these shifts by grounding the story in a kind of bland, muted affectation that encompasses the entire film. Characters speak at a low mumble, the visuals are bleak and graded grey, the edits are slow and blunt. The result is a deceptively simple surface that hides just how much is happening tonally and within the internal lives of the characters.

“Personal Shopper” takes a rather unconventional and tricky approach to storytelling. At a quick glance, it may appear meandering and plotless, but in reality, all of the drama happens within Stewart’s Maureen. It’s tightly focused on tracking her emotional progression as she tries to grapple with all of these feelings, which Stewart, who thrives in muted tones and understated characters, conveys in a brilliant performance. Maureen is a truly perfect character for Stewart — the kind of person whose stony and careful exterior is just a fragile cover for the deep well of feelings underneath.

Despite Stewart’s strength as an actress and Assayas’s clear understanding of plot and character mechanics, “Personal Shopper” is, well, a bit of a slog to get through. For one thing, it’s self-indulgent: For example, about halfway through the film, we’re treated to a nearly ten-minute sequence of Maureen anxiously answering and receiving texts from the unknown number on a train, mostly shot in tight shots of the phone itself. It’s so long and slow that you can’t help thinking there had to have been a better way to convey this information.

Even though its drama is well-articulated through a carefully considered character arc, the movie still feels slow and meandering. It seems more concerned with being artful than with being empathetic. That is, there are some moments of genuine tension in the movie, but those moments are few and far between. We’re left with watching a character experience a lot of emotions, but nothing in the story apart from the character herself exists to enhance those feelings. I don’t suppose that it’s a particularly bad thing for a story to rely on an actor’s performance, but “Personal Shopper” would be absolutely nothing if not for Stewart’s charisma. There are no external conflicts, no ways for the audience to feel what Maureen is feeling — only ways for us to see it. To make a story resonate, storytellers need to work a lot harder than simply telling us what happened. As it stands, “Personal Shopper” gives its audience no reason to care. It’s a shame, too — it could have been beautiful.